Require Gradle 4.9 as minimum version (#32200)

Do the check in the build plugin as well to be more informative to users
of  build-tools.

We use APIs specific to Gradle 4.9 so earlier will not work.

buildSrc/build.gradle

@@ -25,8 +25,8 @@ plugins {
 
 group = 'org.elasticsearch.gradle'
 
-if (GradleVersion.current() < GradleVersion.version('3.3')) {
-  throw new GradleException('Gradle 3.3+ is required to build elasticsearch')
+if (GradleVersion.current() < GradleVersion.version('4.9')) {
+  throw new GradleException('Gradle 4.9+ is required to build elasticsearch')
 }
 
 if (JavaVersion.current() < JavaVersion.VERSION_1_8) {

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -67,6 +67,9 @@ class BuildPlugin implements Plugin<Project> {
                 + 'elasticearch.standalone-rest-test, and elasticsearch.build '
                 + 'are mutually exclusive')
         }
+        if (GradleVersion.current() < GradleVersion.version('4.9')) {
+            throw new GradleException('Gradle 4.9+ is required to use elasticsearch.build plugin')
+        }
         project.pluginManager.apply('java')
         project.pluginManager.apply('carrotsearch.randomized-testing')
         // these plugins add lots of info to our jars